1. Field of the Invention
The present invention relates to a cast part which is made of an aluminum alloy, and in particular, to a fuel tank which is made of an aluminum alloy.
2. Description of the Related Art
One important factor in improving the mileage and running properties of motorcycles and four-wheeled vehicles is to achieve light-weight bodies. Light-weight motorcycles and four-wheeled vehicles may be achieved by forming their component elements by using light-weight materials. Conventionally, many component elements, including the frame and chassis, have been formed of iron or steel containing iron. On the other hand, proposals have been made in the recent years to use aluminum (which has a smaller specific gravity than that of iron) as a structural material. Since aluminum has a density which is about ⅓ of that of iron, use of aluminum or aluminum alloys as the materials for component elements is considered to achieve a substantial reduction in the mass of a motorcycle or four-wheeled vehicle.
Japanese Laid-Open Patent Publication No. 8-232789 discloses a fuel tank for an automobile, which is made of aluminum or an aluminum alloy. This fuel tank includes an upper box member and a lower box member which are formed via drawing, which are then subjected to brazing to form a box-shaped closed space.
When producing such an aluminum or aluminum alloy part, press forming may be used to prepare aluminum or aluminum alloy plates as necessary, and these plates may be welded together to make a part with a desired shape. However, use of plates has a problem in that a plurality of plates must be prepared for the part, thus resulting in an increased number of plates. Moreover, the plates must be joined at many places and aluminum is generally more difficult to weld than steel. This illustrates the problem of high production costs when making an aluminum or aluminum alloy part from plates.
In order to solve such problems, it may be possible to use casting to form the part. However, casting requires a good melt flow, which imposes constraints on the aluminum composition for making the part. Specifically, in order to use casting to form a part with an aluminum alloy, it is necessary to add a predetermined amount of silicon to aluminum in order to achieve a good melt flow. However, the addition of silicon decreases the ductility and toughness of the part which is produced through casting. This is especially problematic in a fuel tank for a motorcycle due to the following reasons. A fuel tank for a motorcycle is often exposed to the exterior of the motorcycle, and therefore in case of falling, etc., the fuel tank is likely to receive a considerable impact. Therefore, a fuel tank for a motorcycle is required to have good ductility and toughness so as not to have any fissures or cracks after application of a considerable impact, which may allow the fuel inside the tank to leak therethrough.
As methods of adjusting the hardness of a metal member without changing its composition, heat treatments such as quenching and annealing have long been known. For example, Japanese Laid-Open Patent Publication No. 9-3610 discloses improving ductility by performing a heat treatment for an aluminum die-cast article that contains silicon. However, according to the studies of the inventors of the present application, it has been found that, in such a heat treatment, age hardening significantly affects the physical properties of the die-cast article which has been subjected to the heat treatment.